Hot pick-up adhesives are low viscosity hot melt adhesives that exhibit tack at elevated temperature and are substantially tack-free when cool. The adhesive can be applied to containers such as cans or jars, and used to pick up one container label from a stack of labels or a label cartridge in order to initiate the labeling process. The container can be used at low, ambient or elevated temperature.
Current hot pick-up adhesives are made with natural rosin materials. These rosin materials are natural products typically derived from tree or plant sources. The natural rosins are generally polycyclic natural products, carboxylic acids or esters, and are considered to be irritants or materials that can provoke an allergic response in users that come into contact with this material at elevated temperature. These materials, both from their composition and from thermal decomposition by-products, have been known to cause at least irritation and allergic responses from human subjects.
Hot pick-up adhesives are typically used with roll-through labeling machines such as those manufactured by Mateer Burt in Exton, Pa. In the roll-through labeling process, a continuous series of containers are rolled over a small glue pot, heated from about 250° F. to 275° F., and filled with melted hot pick-up adhesive. Rotating wheel applicator disks dip into the pot, pull adhesive out, and apply small dots of adhesive onto the surface of the container as it rolls over the applicator wheels. Other machines can be used but these are used as exemplary of the process. After the dots, lines or other shapes of applied hot pick-up adhesive are formed, the container continues to roll through the labeling machine and rotate until the dots of hot adhesive contact and adhere to the end of the top label in the label cartridge. The hot pick-up adhesive has enough adhesion to the label and enough cohesive strength while still molten to pull the top label from the stack of labels and wrap the label around the container as the container continues to roll through the labeling machine. After the container has made one full rotation, the label will be totally wrapped around the container and adhesively over-lap itself where it is permanently adhered with another adhesive material sealing the label to the container exterior.
The hot pick-up adhesive needs the right balance of fluidity, hot tack, and cohesive strength when it first contacts the label and picks it up from the label cartridge. If it is too fluid it will not have sufficient hot tack to pull the label from the cartridge, but if it has too little fluidity, it will not be able to wet-out and adhere to the label or container. Since hot pick-up adhesives are thermoplastic materials their fluidity and hot tack can change substantially with temperature. The hot pick-up is hot and fluid when it is first applied to the container by the applicator wheels. As the container rotates through the labeling machine the hot pick-up begins to cool and it sets and becomes less fluid and builds hot tack. It is important that the hot pick-up have the correct rate of set so that it has sufficient fluidity and hot tack at whatever temperature it has cooled to when it attempts to pick up the label.
Clearly, a substantial need exists for a high quality hot pick-up adhesive that obtains equal or better adhesive properties than the prior art materials, but also provides a hypoallergenic material that is not an irritant to people who come into contact with the adhesive during adhesive manufacture or container labeling.